To put the iPhone into recovery mode so you can do an iTunes firmware restore follow these steps:
Step One
Open iTunes and connect the iPhone to your Mac.
Step Two
Press and hold the Home button and the Sleep/Wake button at the same time.
Step Three
As soon as the screen goes black release the Sleep/Wake button. Continue holding the home button until you iTunes pops up a message telling you that it has detected an iPhone in recovery mode.
The iPhone will look like this:
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